1. Field of the Invention
The present invention relates to a magneto-optical recording medium to be used for information recording and a writing and readout method therefor, and a writing and readout apparatus.
2. Discussion of Background
Magneto-optical recording media are media designed to record information in a magnetic layer as a magnetization direction. They are rewritable with high densities and of low costs, and they are used, for example, as information recording media for e.g. external memory apparatus for computers or for apparatus for recording and replaying music. Among them, a magneto-optical recording medium employing a writing layer made of an amorphous alloy of a rare earth metal or a transition metal, exhibits excellent properties. A magneto-optical disk having a large recording capacity is practically available even now, but in view of the ever increasing quantity of information of the society, modification for a larger capacity is desired. The limit in the recording density of a magneto-optical disk is usually determined by the size of the spot of the readout laser beam. The size of the spot can be made smaller, as the wavelength of the laser is shorter. Accordingly, a study has been made for shortening the wavelength of the laser but with difficulty, and use of a short wavelength laser is a factor for a high cost. On the other hand, attempts for so-called super resolution have been made in recent years, in which it is attempted to obtain a resolution higher than the level determined by the wavelength of the laser, by various measures.
As one of such attempts, a system for magnetically induced super resolution (hereinafter referred to also as MSR) has been reported which employs an exchange coupling force among magnetic layers of a multi-layered structure in a magneto-optical disk.
This system consists essentially of a layer for information recording (a writing layer) and a layer for information readout (a readout layer), whereby writing is carried out against the writing layer, and at the time of readout, the magnetization direction of the writing layer is transferred to the readout layer, and the magnetization direction of the readout layer is readout. Usually, the transferability to the readout layer is controlled by the temperature for heating a magnetic layer of the writing layer or the like.
According to this system, the temperature distribution in a readout laser beam spot is utilized to modify the magnetic domain of the readout layer, whereby the waveform interference of readout signals can be reduced, and high density recorded information can be readout with good quality.
The MSR system includes, for example, one utilizing magnet static coupling and one utilizing exchange coupling.
As one of the MSR systems employing magnet static coupling, the present inventors have proposed in JP-A-7-147029 a system so-called xe2x80x9creversal type MSRxe2x80x9d which employs a medium comprising mutually exchange-coupled three layers i.e. a readout layer having a small coercivity, a switching layer having a low Curie temperature and a writing layer having a high Curie temperature and a large coercivity.
Here, each of the readout layer and the writing layer is made of an alloy of a transition metal and a rare earth metal. Magnetization of each layer is determined by the sub-lattice magnetization of the rare earth metal and the sub-lattice magnetization of the transition metal.
A composition in which the magnetization of the rare earth metal and the magnetization of the transition metal cancel out each other, is called a compensation composition, and a composition containing the transition metal in a larger amount than the compensation composition is called transition metal magnetization dominant (hereinafter referred to also as transition metal rich or TM rich), whereby the overall magnetization agrees to the transition metal magnetization. A composition containing the rare earth metal in a larger amount than the compensation composition is called rare earth metal magnetization dominant (hereinafter referred to also as rare earth rich or RE rich), whereby the overall magnetization agrees to the rare earth metal magnetization.
In the reversal type MSR system, at a low temperature portion within the spot of readout laser beam, the readout layer and the writing layer are exchange-coupled via the switching layer. In the exchange coupling, the direction of the sub-lattice magnetization of the readout layer agrees to the direction of the sub-lattice magnetization of the writing layer.
On the other hand, at a high temperature portion, the temperature of the switching layer exceeds the Curie temperature of the switching layer, whereby the exchange coupling of the readout layer and the writing layer will be cut off. Accordingly, in the relation between the readout layer and the writing layer, the magnet static coupling tends to be governing, whereby the magnetization direction of the readout layer agrees to the magnetization direction of the writing layer. If both the readout layer and the writing layer are TM rich, the magnetization directions of the readout layer and the writing layer will always be the same.
However, when the dominant magnetizations differ, for example, in a case where the readout layer is RE rich, while the writing layer is TM rich, the magnetization state in the readout layer will be opposite as between a case where the sub-lattice magnetization directions of the readout layer and the writing layer agree to each other and a case wherein the magnetization direction of the readout layer and the writing layer agree to each other.
Namely, when the medium is heated under irradiation with a readout beam, firstly at a low temperature portion, magnetization of the readout layer will appear due to the exchange coupling with the writing layer, and when it is further heated to a high temperature, the exchange coupling is cut off, whereby the magnetization of the readout layer will be reversed.
In high density recording, this reversed magnetization serves to intensify the signal together with the non-reversed adjacent mark, whereby high resolution can be realized.
In a reversal type MSR system, like a magnetic disk, the signal intensity becomes maximum at the edge of the mark (at the boundary of a magnetic domain) rather than the center of the mark (the magnetic domain), whereby the mark edge can easily be detected by detecting the peak position of the signal. Accordingly, there is an economical merit in that readout of a mark length modulation recording signal can be carried out by utilizing an inexpensive signal-detecting circuit which is commonly used in a magnetic disk apparatus.
Further, as another type of a MSR system employing magnet static coupling, a system so-called xe2x80x9cstatic coupled CADxe2x80x9d has been proposed wherein a readout layer which has in-plane magnetization at a low temperature and which becomes a perpendicular magnetization film as the temperature becomes high and as the magnetization becomes small, is employed as the readout layer, a non-magnetic barrier layer is provided between the writing layer and the readout layer, and the magnetization direction of the writing layer is transferred to the readout layer solely by the magnet static coupling force.
In the static coupled CAD system, only at a high temperature, the magnetization of the writing layer will be transferred to the readout layer, and the signal can be readout. This system is excellent in that at a low temperature, no transfer of the magnetization direction of the writing layer will take place, and the layer remains to be an in-plane magnetization film, whereby a so-called low temperature mask will be formed, and the signal interference (cross talk) with the formed adjacent tracks can be minimized.
On the other hand, as a MSR system utilizing only exchange coupling without employing magnet static coupling, a system so-called xe2x80x9cexchange-coupled CADxe2x80x9d is available wherein a readout layer which has in-plane magnetization at a low temperature and which becomes a perpendicular magnetization film as the temperature becomes high and as the magnetization decreases, is used as the readout layer, and the readout layer and the writing layer are exchange-coupled, so that the magnetization direction is transferred to the readout layer.
However, in such an exchange-coupled CAD system, if the readout layer is made to have a composition so that it becomes a perpendicular magnetization film at a high temperature, it becomes difficult to make it a completely in-plane magnetization film at a low temperature due to the strong exchange coupling with the writing layer. And, the boundary between the in-plane magnetization region and the perpendicular magnetization region becomes obscure, and the resolution of the readout signal tends to be low. If it is attempted to make the readout layer an in-plane magnetization film at a low temperature, it will be required to make the layer substantially thick, thus leading to an increase of the production cost of such media and a decrease in the recording sensitivity.
Accordingly, a MSR system utilizing magnet static coupling is superior from the viewpoint of the readout resolution.
In a system (hereinafter generally referred to as static coupled MSR) for transferring the magnetization direction of a writing layer to a readout layer by utilizing magnet static coupling like the above-mentioned xe2x80x9creversal type MSRxe2x80x9d or xe2x80x9cstatic coupled CADxe2x80x9d, it is necessary to increase the leakage flux from the writing layer to the readout layer, which is the source for the magnet static coupling, in order to carry out the readout efficiently.
For this purpose, the magnetization of the writing layer and the readout layer may be increased.
However, it has been found that a new problem will be brought about if such a composition suitable for readout, is employed.
Such a problem will be described with reference to a medium of a reversal type MSR system wherein a readout layer is RE rich, and a writing layer is TM rich.
The magnetization of the writing layer can be made large by shifting the compositional ratio of the rare earth metal and the transition metal in the writing layer substantially to the TM rich side from the compensation composition. However, if the magnetization of the writing layer is made large, the magnetization recorded in the writing layer during the writing, tends to undergo magnetization reversal to an unintended direction.
Namely, in a region of the writing layer wherein the magnetization is consistent in a uniform direction, the magnetic flux from a surface magnetic pole of the writing layer forms a de-magnetization field in the interior of the writing layer. Such a de-magnetization field acts against the surrounding as a force to form reversed magnetization. Accordingly, if a part of the writing layer is heated during the writing, and the coercivity decreases, a strong de-magnetization field from the surrounding writing layer not so heated, will be effective to cause magnetization reversal. Therefore, the magnetization is likely to be shifted to an unintended direction, when no external magnetic field is applied during recording, or against an external magnetic field, even if such an external magnetic field is applied.
As the magnetization given to the writing layer increases, this de-magnetization field simultaneously increases, and the above described magnetization reversal tends to occur during the writing. Accordingly, large magnetization for writing/erasing will be required to overcome the de-magnetization field at the time of writing/erasing.
This is problematic not only in a reversal type MSR system but also in any MSR system utilizing magnet static coupling.
Writing systems for magneto-optical recording media include a light intensity modulation method and a magnetic field modulation method.
The former is a system in which usually, erasing is carried out once to align the magnetization in one direction, and then a writing magnetic field in a reversed direction is applied, and writing is carried out by changing the intensity of irradiated light. The latter is a system wherein the light intensity is maintained to be constant or in a pulse form, and writing is carried out by changing (reversing) the writing magnetic field.
The magnetization reversal due to the de-magnetization field is extremely inconvenient especially for the magnetic field modulation system.
In order to carry out the writing accurately, it is necessary to set the writing magnetic field stronger than the de-magnetization field, as mentioned above. However, if the writing magnetic field is increased, it tends to be difficult to change (reverse) the magnetic field at a high speed, whereby the magnetic field intensity tends to be inadequate.
In order to carry out the writing at a high speed, the magnetic field intensity is obliged to be weak, whereby there will be a problem of an unintended magnetization reversal due to the de-magnetization field.
Also in the light intensity modulation system, the surrounding de-magnetization field is convenient when a writing magnetic domain is to be formed at a region once erased to align the magnetic field in one direction, but if a large de-magnetization field is present excessively, there will be a problem that it tends to be difficult to carry out erasing uniformly.
To reduce the de-magnetization field, magnetization of the writing layer may be made small, and it has been believed that with conventional media, a composition close to the compensation composition is suitable.
However, with such a composition, the leakage flux at the time of readout will also decrease, which adversely affects the readout characteristics on a principle of a system employing a magnet static coupling. Specifically, no adequate transfer of the magnetization direction to the readout layer tends to be carried out.
Namely, in a conventional MSR system employing a magnet static coupling, it has been difficult to satisfy both satisfactory transferability from the writing layer to the readout layer at the time of the readout and a reduction in the writing magnetic field at the time of the writing.
It is an object of the present invention to solve such a problem and satisfy both the satisfactory transferability from the writing layer to the readout layer at the time of the readout and the writing with a low writing magnetic field, and to provide a magneto-optical recording medium excellent in both the readout characteristics and the writing characteristics, particularly a magneto-optical recording medium suitable for a magnetic field modulation system and a writing and readout method therefor, and a writing and readout apparatus.
As a result of a study, the present inventors have found it possible to satisfy both writing with a low writing magnetic field and satisfactory transferability to the readout layer by having the writing layer composed of a plurality of layers having different characteristics.
Namely, the present invention provides a magneto-optical recording medium having a writing layer, a switching layer and a readout layer on a substrate, wherein a magnetization direction corresponding to information is written in the writing layer, and the magnetization direction of the writing layer is transferred to the readout layer by a magnet static coupling force from the writing layer at a temperature of not lower than room temperature, and wherein the writing layer is composed of a plurality of layers which are made of alloys of a rare earth metal and a transition metal and which are exchange-coupled one another, including a layer having rare earth metal dominant magnetization at room temperature and a layer having transition metal dominant magnetization at room temperature.
Here, room temperature is usually meant for 25xc2x0 C.
Further, the present invention provides a writing method for a magneto-optical recording medium, wherein such a magneto-optical recording medium is used, and writing of information thereon is carried out by a magnetic field modulation system.
Still further, the present invention provides a writing and readout apparatus comprising such a magneto-optical recording medium and a flying type or contact type magnetic head.